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Earth’s Crust Isostasy

The outermost part of the Earth consists of a fairly rigid layer called the lithosphere, which con­sists of the crust and the top layers of the mantle. It floats on a yielding layer beneath it, termed the asthenosphere.

The height of the surface of the lithosphere (measured relative to sea level) varies in response to various factors, such as changes in mass caused by the addition or removal of rock and changes in density due to heating and cooling.

These balancing movements of the lithosphere and asthenosphere are known as isostasy, and have provided geologists with much information about the composition of the interior of the Earth.

Isostasy Effects

Various general features of the Earth's surface result from isostasy. For example, continents are higher than ocean floors not because they have more volume but because their constituent rocks are less dense than those of the seabed.

A similar effect can be seen with wood floating in water; the heights of the blocks above water level depend on their thickness and density. Mountains are usually higher than plains because they are underlain by a thick layer of the relatively low-density continental rock.

In contrast, the weight of Green­land's great ice sheet has depressed the landmass beneath it and caused rock surface to become sau­cer-shaped, with its center below sea level.

The asthenosphere is viscous, much like extremely thick treacle, and it therefore does not respond rapidly once a constraint is removed. Full isostatic readjustment may take many thousands of years. For this reason depressions in North America that were caused by the weight of ice during the last ice age remained after the ice melted; they filled with water, creating the Great Lakes.

Another consequence of isostasy is that as ero­sion wears away land and makes it less massive, the land rises up to maintain the equilibrium, and so compensates for the imbalance caused by ero­sion. As erosion cuts deep valleys into a mountain chain, individual peaks may simultaneously rise even though the average surface height as a whole decreases.

Conversely, as a large basin fills with sediment it sinks and effectively deepens so that eventually the sedimentary layer may become thicker than the original depth of water.

Gravity and Isostasy

The force of gravity arises from the pull of the whole Earth, but it varies slightly from place to place because of the unequal local distribution of mass in the Earth's surface layer.

If a mountain were simply extra rocky material rising above the surrounding plain, the force of gravity would be greater on the mountain by an amount that depends on the size of the mountain, its density, and the slightly greater distance to the mountain top from the centre of the Earth. An increase in the acceleration due to gravity occurs across mountain ranges, but not as much as would be expected because the extra mass rising up is largely compensated for by the greater depth to which the base of the mountains extends down into the denser supporting medium (the astheno­sphere).

Some mountains, such as the Andes and the Himalayas, are still being formed, and because isostasy takes time to come into effect their "growth" downwards into the asthenosphere is not complete.

Isostasy was originally discovered through its effect on gravity, and variations in gravity are still used to measure the degree of iso­static compensation.

Ocean-spreading Ridges

Isostasy also helps to explain the shapes of sub­marine "mountain ranges" such as the Mid-Atlan­tic Ridge. Such oceanic ridges form where two crustal plates are moving apart and molten rock wells up from the mantle to fill the gap. These ridges are high because they are less dense than the surrounding material.

Over the surface of the ocean gravitational attraction varies very little, despite the varying depth of water below, demon­strating nearly perfect isostatic compensation. The ridges must, therefore, be supported on rock that is less dense, although this has much the same chemical composition as the rock on either side which the ridges continually replace as the plates move apart. The ridges are probably less dense also because they are hotter due to their upwelling activity.

An oceanic plate is formed from the rigid, rela­tively cool (less than 1,200°C) topmost part of the mantle. As a plate moves away from a mid-ocean ridge, it cools and thickens, and settles down­wards.

The thickness of the ocean floor beyond a ridge crest should increase with its age. Areas of similar age in different spreading ridges have a similar profile regardless of the rate of spreading. If it were possible to reheat the plates they would expand and rise again to their original heights.

Lithosphere and Asthenosphere

For ocean plates, the chief difference between the lithosphere and the asthenosphere is one of tem­perature, although chemical processes below a spreading ridge (where magma forms) cause the upper few kilometers of the lithosphere (the crust) to be of lighter material.

The continental lithos­phere is more complex. Much of its lightness is due to a crustal layer, which is less dense than the oceanic crust. Most continental crust is between 30 and 50km thick, but can be twice as thick beneath great mountain ranges. It is this thickness that provides much of the buoyancy that supports the mountains. The nature of the material below the continental crust is not known with certainty, although it has been established that the thickness of the continental lithosphere is about 100km.